In the combustion of a fuel, such as coal, oil, peat, waste, etc. in a combustion plant, such as those associated with boiler systems for providing steam to a power plant, a hot process (or flue gas) is generated. Such a flue gas will contain, among other things, carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen disulfide (H2S2), and/or carbonyl sulfides (OCS). A variety of methods and technologies exist in order to remove the pollutants from the flue gases. One method for the removal of carbon dioxide from a post-combustion flue gas is the Chilled Ammonia Process (CAP).
With this the absorption of carbon dioxide from the flue gas is achieved by contacting a chilled ammonia ionic solution (or slurry) with the flue gas. For example, the flue gas is brought into countercurrent contact with an absorption solution, for example, a liquid ammonia-based solution or slurry, in an absorber. In the absorber, a contaminant-free, i.e., “lean” gas stream is formed and a contaminant-rich absorbent, i.e., a “rich” solution is formed.
After having absorbed the contaminants, the “rich” solution is sent to be “regenerated”, where heat and pressure are used to separate the absorbent solution from the contaminants in order to create an absorbent solution that can be re-used in the absorber to capture further contaminants.
One of the highest cost penalties of the absorption-capture type systems is the regenerator. The heat and energy required to release the contaminants from the solution heavily burdens the rest of the plant.
For example, the regeneration process is generally performed at elevated pressure, such as 10-150 bara, and temperature, such as 50-200° C. The pressure is generally created by means of a high pressure pump arranged in connection with the regenerator. Regenerators typically operate at a high internal pressure and require the use of high-pressure steam to sufficiently heat the ammonia-based solution to release the carbon dioxide from the ionic solution. Under these conditions, (i.e., high pressure and temperature), nearly all of the absorbed carbon dioxide is released into the gas phase in order to form the CO2-rich gas stream.
The gas phase may also comprise a minor portion of gaseous NH3 (i.e., ammonia slip), which can be condensed and returned to the capture system for use in capturing further CO2 from the gas stream.
In some cases, steam can be used for heating from other parts of the power plant, but then that is at a cost to the plants' electric output and production.
Additionally, the regenerator may also include a reboiler for heating. While this may reduce the need for steam, this further taxes the plant by requiring the additional energy to run the reboiler.
As high-pressure steam stores/contains a significant amount of thermal energy, it is usually considered a valuable commodity. Thus, heating an ammonia-based solution to facilitate release of the CO2 is often an expensive process.
Further, the CO2-rich gas stream produced by a regenerator is typically stored/transported at a pressure at/or near the internal operating pressure of the regenerator. The high pressure of the CO2-rich gas stream produced by a regenerator operating at a high internal pressure may provide benefits such as reducing the amount of ammonia in the CO2 product, but also minimizes the downstream compressor duty. While these benefits are useful, they come at the cost of requiring high-pressure steam and increasing the overall electric demand, which penalizes the power plant output.
In some instances, it may be that the customer wants the CO2 product at a lower pressure for use in applications/processes requiring CO2 at lower pressures. To meet these needs by reducing/lowering the pressure of CO2-rich gas stream (as produced by a regenerator operating at a high internal pressure) is typically an expensive process.
Furthermore, when operating the regenerator at a lower pressure, the ammonia slip increases substantially, thus increasing the operating costs and ultimately not reducing steam consumption because of the increased duty of the stripper.
What is needed, therefore, is an improved system and method for recovering ammonia from a gas stream.